We present a computationally efficient and fast semi-numerical technique for simulating the Lyman-$\alpha$ (Ly$\alpha$) absorption optical depth in presence of neutral hydrogen "islands" left over from reionization at redshifts $5 \lesssim z 6$. The main inputs to analysis are (i) photon-conserving model ionized regions during (named SCRIPT) along with prescription shadowing by islands (ii) flu...